How to Avoid Color Changes in Screen Printing

It's true: Colors do change with screen printing. Find out why and what you can do about it.

It's true: Colors do change with screen printing. Find out why and what you can do about it.
 

Our definition of printing is "replicating original artwork by laying down a controlled thickness of ink." Maintaining edge definition is also a key part of replicating original artwork, but controlling the thickness of the ink deposit is even more critical--and more difficult.

In graphics screen printing, ink-deposit thickness has a major effect on the color that will be visible to the viewer. In industrial printing, the ink-film thickness can affect the functionality of the print. This month, we'll focus on ink-deposit thickness in graphics applications by considering some of the variables you can control in your plant to reproduce color as consistently as possible. Our discussion won't address color separations or other artwork considerations, although these aspects of production obviously influence color, too.

Ink Condition

The ink is the most influential component in determining whether or not you can correctly match a color. The ink has to be correct at the start of the job, and it must stay that way throughout the run. If the ink contains any fluids that are likely to evaporate, the color can change as this evaporation occurs. The most obvious occurrences of this phenomenon happen with solvent-based ink systems. As the solvents evaporate from the ink on screen, the concentration of pigments in the ink increases, resulting in a color change. Inks containing water (conventional water-based inks and water-miscible UV formulations) are similarly affected, as are any inks that are dried and cured by heat/evaporation.

Because they are nonevaporative, pure UV inks and plastisols are the only ink systems that don't introduce this variable. However, the temperature during curing can affect the color of these inks, as we'll discuss later.

You can deal with ink-induced color shifts in a number of ways, but the two things that you don't want to do are add solvents to the ink on the screen, or remove ink from the screen and put it back in with freshly mixed ink. Both of these approaches are guaranteed to change the color of your prints. Treat any ink you remove from the screen as waste ink. This includes UV inks and plastisols, which may experience color shifting from dust and other contaminants they collect on the screen.

Mesh selection

Those who think that mesh count is the only variable that affects the ink-deposit thickness are in trouble. Both the thread diameter and weave (plain or twill) can substantially affect the ink deposit as well. Mesh suppliers provide a number of specifications as to how their products will perform, and one of the most important is theoretical ink volume (listed by some manufacturers as "ink thickness" or "theoretical ink deposit"). This indicates the amount of ink that will pass through the mesh under given printing conditions, generally shown in cubic centimeters per square meter. On the specification sheet you get from your mesh supplier, you'll see a single value (in microns) that shows the wet ink-deposit thickness for each mesh they sell.

You can use this specification in a number of ways--for example, to calculate the amount of ink you need for a job (see "How Much Ink Do You Need?" Screen Printing, April 2000, page 36). Most importantly, you can see the dramatic difference in ink deposit between two seemingly similar meshes. Take two fabrics with a 390-threads/in. (150-threads/cm) mesh count, one with a 31-micron thread diameter and one with a 34-micron thread. The two meshes will have wet ink-film thicknesses of 11 and 6 microns, respectively. Simply stating "390 mesh" on your work order could lead to a tremendous difference in ink-film thickness and, consequently, a large variation in color as well.

The weave of the mesh isn't as big an issue today as it was a decade ago. You are far less likely today to get a twill-weave mesh when you were expecting a plain weave, although it still happens occasionally. Typically, a twill weave will vary 10% in the theoretical ink volume of a comparable plain-weave mesh, not to mention the fact that it would hamper your ability to print fine details.

Mesh tension

Low tension will cause the mesh to lift too slowly off the ink on the substrate during the printing cycle. Consequently, the mesh will tend to retain ink and produce a mottled effect on subsequent prints, conditions that cause the color to change. To overcome insufficient screen tension, you must increase the off-contact distance, and once you do that, you must increase squeegee pressure as well. This, in turn, will affect the amount of ink that flows through the mesh during the print stroke, resulting in further color change. For these reasons, always use properly tensioned screens.

Some printers will use the peel function on their press to overcome insufficient mesh tension. This feature raises the frame as the squeegee traverses the stencil, lifting the mesh away from the ink film. The important thing about peel and off-contact settings is that once they have been set for a job, they can't be changed during the print run. This applies with multicolor presses as well as single-color machines. Altering the settings mid-run will change the image size and throw process colors out of registration with one another.

Squeegee settings

Unless you are printing by hand (in which case you have to use your squeegee to flood the screen), the squeegee carries out just two functions--it brings the stencil into contact with the substrate, and it causes ink to flow into the mesh. If a squeegee is set up incorrectly and does anything else, it will have an adverse effect on the process.

Bearing this in mind and assuming that the angle, pressure, hardness, and dimension of the squeegee are all correct, two additional factors influence the ink deposit. The first is squeegee speed. As a rule of thumb, a fast squeegee will lay down less ink than a slow one. The second and probably most insidious factor is the wear on the squeegee edge that occurs during the print run. The edge of the squeegee will gradually round off over the course of a job, reducing its effective angle of attack. This, in turn, increases the flow of ink into the mesh, resulting in a thicker ink film and a change in color.

To help prevent the edge from deteriorating, keep squeegee pressure to a minimum. If you are printing a very long run, prepare several squeegees identically and have them ready and waiting in their holders when you start the job. Change the squeegees at regular intervals, before any edge wear becomes apparent. Don't even think of taking the squeegee off, sharpening it, it and putting it back on the press immediately.

Floodbar settings

Many printers do not realize that the floodbar can radically affect how much ink is printed. The floodbar's job is to coat the mesh with a consistent amount of ink. It doesn't just prevent ink from drying in the mesh--it plays a critical role in the consistency of the entire printing process. Altering the floodbar's pressure or angle can result in an excessive or insufficient amount of ink on the mesh. The speed of the floodbar is important as well: If it's too slow, the mesh will be overloaded with ink; too fast and the mesh will suffer from "ink starvation."

The floodbar's edge has an impact on its performance, but this edge won't wear as quickly as a squeegee's. You're much less likely to see color shifts resulting from changes in the floodbar edge than you are from making adjustments to the floodbar during the print run.

Machine settings

Process control is the key. Consistent setups mean consistent color. Variations in job settings will send color out of control. The classic problem occurs when printers change shifts and the incoming press operator alters the machine settings to suit his style. This causes the color to change. Modern presses that use computerized controls help reduce this possibility by making it easier to set up the press to exact specifications, maintain those settings throughout the print run, and recall them if the job is repeated later.

Substrates

An often-ignored aspect of screen printing is the consistency of the substrate. Paper, board, and plastics are generally produced in batches, and a good-quality supplier will maintain the surface finish of the substrate throughout the batch. But this is not always the case. Sometimes changes in manufacturing will alter the color of the substrate, as well as its gloss level and absorbency. When any of these things changes, it leads to a color shift even if nothing in the printing process has changed.

As a screen printer, you face real difficulties because of the range of different substrates onto which you might be asked to print a promotion. You may be asked to match a litho-printed image, but without careful process control, you have no chance. Your probability of success increases dramatically if you use such tools as a spectrophotometer for line art and a densitometer for process color. But if you rely on the traditional method--flying by the seat of your pants--the variables will be so great that the process will be completely out of control.

Viewing light

Nature has played an unkind trick upon us, making the same color appear different under different lighting conditions. The human eye is remarkably sensitive to these changes. You can minimize this variable by making sure the pigment and pigment level are precisely the same throughout the print run. Try to mix all the ink you'll need for the job in one batch. Don't change ink systems or brands during the middle of a run. Color perception is a very complex area. To gain the best possible control, be sure you have a closed loop of ink manufacturing, mixing, proofing, and color measurement.

Drying

We have all suffered color changes caused by incorrectly set dryers at one time or another. A common example when printing paper or board is having white image elements turn yellow due to excessive dryer temperatures. UV-based lacquers also have a tendency to yellow. The printers that suffer most from color changes during drying (or, in their case, firing) are glass and ceramics decorators. Their pigments can change completely from printing to firing. The final colors are affected not only by the firing temperature, but also by oxidizing or reducing qualities of the atmosphere within the firing zone.

True colors

The difficulty in reproducing colors accurately and consistently is one of the things that makes screen printing so interesting and challenging. Your odds in achieving correct color only improve when you take the variables of the printing process out of the picture. This discussion highlighted some of the key areas where you should focus your attention. Learn to control these elements of the screen-printing process, and you'll be able to deliver quality color time after time.
 

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